This invention relates to a biaxially oriented polymeric sheet having attenuated blocking properties and a method for attenuating the blocking properties of biaxially oriented polymeric sheet.
It is well known that biaxially oriented sheeting manufactured from polymeric materials has the tendency to block or adhere to itself in rolls or when stacked in sheets. Certain polymers, such as those of the styrene family of plastics, have to be biaxially oriented in sheet form so as to develop the necessary toughness for most practical uses having high utility. The process of orientation wherein a material is extruded into a thick sheet and then stretched biaxially to 1 to 30 mils has a tendency to glaze the surface of the sheet giving it a high gloss and good see-through properties. However, the extreme smoothness of the sheet allows the sheeting to contact itself very intimately promoting coherence and blocking.
The problem of blocking in biaxially oriented polymeric sheeting, particularly in the range of thickness of 1 to 30 mils, is a very serious problem. Rolls block and are difficult to unroll by the fabricator. Stacked sheets adhere to each other and will not feed well into fabricating equipment. Finished articles formed from the sheet, e.g. lids, dishes, etc., will not disengage readily in automatic vending machines.
A great number of additives have been used in sheet compositions to overcome blocking such as waxes, oils, lubricants, plasticizers, etc., which can migrate to the surface of the sheet and reduce the coherence of the sheet to itself. Such materials have not been entirely satisfactory and have presented problems in that such additives can produce haze or are temporary in nature. Further, being on the surface, such additives and coatings can produce problems in treating the fabricated sheet such as printing, labeling, sealing, etc.
Many methods have been proposed to prevent blocking, none of which is entirely satisfactory. External methods such as applying coatings of waxes, oils and lubricants have been used. Such materials have been coated during sheet production operations but are often only temporary in their effect as they can migrate into the sheet or be removed in fabrication. External treatment requires additional manufacturing steps creating cost and quality control problems. As discussed, such treatment can create problems in subsequent fabrication, e.g. printing, heat sealing, etc.
In accordance with the present invention, it has been found that the blocking of biaxially oriented polymeric sheeting can be attenuated, particularly those of polyvinylidene aromatic polymers or copolymers, by incorporating a grafted diene rubber in such polymers. These grafted diene rubbers were dispersed substantially throughout the polymer have been found to attenuate the blocking properties of said sheeting on a permanent basis for the service life of the sheeting. Such grafted diene rubbers do not migrate out of the sheet nor are they removed by exposure to water or other fabricating means.
Such grafted diene rubbers when incorporated in the sheeting do not create a haze problem or subsequent fabricating problems for the sheeting. Finally, such grafted diene rubbers can be readily dispersed in the polymeric material and formed into a biaxially oriented sheet by simple processing means requiring a minimum of processing steps giving a sheeting having great utility.